Extrusion equipment for the manufacture of plastic pipe is well known and used for forming pipe of different cross sections and diameters. For high strength plastic pipe, the pipe wall has multiple sections to improve the performance of the pipe with respect to radial compressive forces or to meet other design criteria. For buried pipe installations, the most common failure mode is inward buckling. Various forms of corrugated double wall pipe, ribbed pipe, as well as varying shaped solid wall pipes, are available for different applications. Double walled corrugated pipe is available in different diameters and is commonly used for drainage and other applications.
Pipe molding equipment includes an extruder in combination with a corrugator or moving mold block system to define a moving mold tunnel. The extruder includes an extrusion head that cooperates with the mold blocks to determine the shape of the pipe. Examples of such systems are shown in our U.S. Pat. Nos. 5,516,482; 6,155,813; 6,054,089 and 6,399,002.
The moving mold tunnel includes cooperating pairs of driven mold blocks that form a portion of the moving mold tunnel upstream of the extrusion head and move downstream of the extrusion head until the extruded plastic has cooled sufficiently to maintain its shape. The mold blocks then separate from the pipe and return to an initial position. The mold blocks are commonly connected to an endless drive conveyor to form opposed mold block trains. The mold blocks can also be driven to form a pulsating corrugator (see our U.S. Pat. Nos. 6,905,325 and 7,104,777). Various arrangements for forming a moving mold tunnel are commonly used in single and double wall pipe manufacture.
With the manufacture of large diameter pipe having a diameter of 50 cm or greater, the speed of the moving mold tunnel is relatively slow and the mold blocks defining the mold tunnel have considerable mass. The manufacture of large diameter pipe requires a high capacity extruder to provide sufficient plastic for forming the pipe. A sudden shutdown of the extrusion system is difficult as the high pressure molten plastic of the extruder is difficult to stop and commonly extrudes into and fills any voids between the mold blocks and the cooling plug which voids under normal operation would be empty.
This excess plastic can damage the molding system and/or significantly complicate the steps necessary to return the system to an operating mode.
In a sudden unexpected shutdown of a molding system for large diameter pipe, the high pressure and output of the extruder is partially offset by the excess plastic filling and setting in the mold block cavities between the mold blocks and the cooling plug. If this excess plastic sets it adheres to or binds with the cooling plug and mold blocks often preventing further movement of the moving mold tunnel. In many cases, the mold blocks must be removed from the associated drive arrangement and the set plastic removed from the mold blocks and cooling plug. This is a time consuming, labor intensive process that significantly impacts the manufacturing cost and associated loss in production time and/or production capacity.
The present invention is directed to a system and method that alleviates a number of these problems.